1. Field of the Invention
The present invention relates to an organic electroluminescent device having an organic compound layer that generates light upon application of an electric field. More particularly, the present invention relates to an organic electroluminescent device that emits light using alternating current bias.
2. Description of the Related Art
Compared to inorganic compounds, organic compounds have more various material systems and possibilities for synthesizing organic materials to have advanced various functions through appropriate molecular design. Further, things made from the organic compound have characteristics of being flexible, and moreover, having workability by polymerization. In light of these advantages, in recent years, the technique of photonics and electronics employing functional organic materials have been attracted attention.
The technique of photonics utilizing optical properties of organic materials has already played an important role in contemporary industrial technology. For example, photosensitive materials such as a photoresist have become indispensable for a photolithography technique that is used for the micro machining of semiconductors. In addition, since the organic compounds themselves have properties of light absorption and light emission caused by the light absorption (fluorescence or phosphorescence), they are also well suited to light emitting materials for laser pigments or the like.
On the other hand, since organic compounds do not have carriers themselves, they have essentially superior insulation properties. Therefore, with respect to the technique of electronics using electrical properties of organic materials, the almost organic compounds have been conventionally used as insulators such as insulating materials, protective materials, or covering materials.
However, there is means for applying large amounts of current to the organic material that is essentially insulators. The means is increasingly coming into practical use in the electronics field. It can be broadly divided into two categories.
One of the means, as represented by conductive polymers, is that an acceptor (electron acceptor) or a donor (electron donor) is doped to give carriers to the π-conjugate system organic compound (Reference 1: Hideki Shirakawa, Edwin J. Louis, Alan G. MacDiarmid, Chwan K. Chiang, and Alan J. Heeger, “Synthesis of Electrically Conducting Organic Polymers: Halogen Derivatives of Polyacetyrene, (CH)x”, Chem. Comm., 1977, 16, 578-580). Since the carriers expand to a certain area with increased the amount of doping, the dark conductivity will also rise along with this, so that large amounts of current will become flow in the organic material.
A part of the means for applying current to the organic material by doping an acceptor or a donor to improve the dark conductivity has already been applied in the electronics field, for example, a rechargeable secondary battery using polyaniline or polyacene, or an electric field condenser using polypyrrole.
The other means for applying large amounts of current to the organic material is utilization of an SCLC (Space Charge Limited Current). The SCLC is the current that starts to flow by injecting and transferring a space charge from the outside. The current density of the SCLC is expressed by Child's Law, i.e., Formula 1 in the following. In the formula, J denotes current density, ∈ denotes relative permittivity, ∈0 denotes permittivity of vacuum, μ denotes carrier mobility, V denotes a voltage, and d denotes a distance to which the voltage V is applied:J=9/8·∈∈0μ·V2/d3  Formula 1
Note that Formula 1 that expresses the SCLC does not assume at all carrier-trap generated when the SCLC flows. The electric current limited by carrier-trap is referred to as TCLC (Trap Charge Limited Current) and in proportion to power of the voltage. The rate of both SCLC and TCLC are determined by bulk. Therefore the SCLC is regarded the same as TCLC hereinafter.
Here, for comparison, the current density when Ohm current flows according to Ohm's Law is shown in Formula 2. σ denotes a conductivity, and E denotes an electric field strength:J=σE=σ·V/d  Formula 2
In Formula 2, since the conductivity σ is expressed as σ=neμ (where n denotes a carrier density, and e denotes an electric charge), the carrier density is included in the factors controlling the amount of current. Therefore, unless increase of the carrier density by doping as described above is attempted to an organic material having a certain degree of carrier mobility, the Ohm current will not flow in the organic material in which carriers hardly exist usually.
However, as shown in Formula 1, the determination factors of SCLC are the permittivity, the carrier mobility, the voltage, and the distance to which the voltage is applied. The carrier density is irrelevant. In other words, it is possible to inject a carrier from the outside and to apply the current to the organic material even an organic material is an insulator having no carriers by making the distance d sufficiently small and by using a material having significant carrier mobility μ.
When this means is used, the amount of current in the organic material is as much as or more than that of a common semiconductor. Thus, an organic material with high carrier mobility μ, in other words, an organic material capable of transporting potentially a carrier, can be referred to as an “organic semiconductor”.
Incidentally, organic electroluminescent devices (hereinafter, organic EL devices) achieve a striking prosperity in recent years as photoelectronic devices which utilize both photonics and electrical qualities of functional organic materials among organic semiconductor devices which use the SCLC.
The most basic structure of the organic EL device was reported by W. Tang, et al. in 1987 (Reference 2: C. W. Tang and S. A. Vanslyke, “Organic electroluminescent diodes”, Applied Physics Letters, Vol. 51, No. 12, 913-915 (1987)).
The device reported in Reference 2 is a type of diode element in which electrodes sandwich an organic thin film to have a total thickness of approximately 100 nm that is formed by laminating a hole-transporting organic compound and an electron-transporting organic compound. For the electron-transporting compound, a light emitting material (fluorescent material) is used. By applying voltage to the device, light-emission can be extracted to outside as a light emitting diode.
The light-emission mechanism is considered as follows. By applying the voltage to the organic thin film sandwiched by the electrodes, the hole and the electron injected from the electrodes are recombined inside the organic thin film, and formed to be an excited molecule (hereinafter, referred to as a “molecular exciton”), and then, light is emitted when this molecular exciton returns to its base state.
Note that, singlet and triplet excitons formed by the organic compound can be utilized. Since the base state is normally the singlet state, the light emission from the singlet excited state is referred to as fluorescent light, and the light emission from the triplet excited state is referred to as phosphorescent light. In this specification, the light emission from either excited states will be described.
In the above-described organic EL device, the organic thin film is normally formed into a thin film to have a thickness of about 100 to 200 nm. Further, since the organic EL device is a self-luminous device in which light is generated in the organic thin film itself, a backlight that is used in a conventional liquid crystal display is not necessary. Therefore, the organic EL device has a great advantage of being able to be manufactured to be ultrathin and lightweight.
Further, in the thin film having a thickness of about 100 to 200 nm, for example, the amount of time for injecting and recombining of carriers is approximately several tens of nanoseconds taking into consideration of the carrier mobility of the organic thin film. Even if the process of carrier's recombination and light emission, light emission can be achieved within on the order of microseconds. Therefore, extremely quick response time can be included in advantages of the organic thin film.
Because of the above-mentioned properties of thin and lightweight, the quick response time, and the like, the organic EL device is attracted an attention as a next generation flat panel display device. Further, since the organic EL display has a high level of visibility from its property of self-luminous and a broad visible range, the organic EL device is expected to be used for display screens of portable devices.
An organic EL device is the device that utilizes means of applying SCLC to an organic semiconductor, but the SCLC intensifies the deterioration of the organic semiconductor function. As to the organic EL device, it is known that the device lifetime (half-life of the luminance) is reduced inversely proportional to the initial luminance, in other words, the amount of current flowing. (Reference 3: Yoshiharu SATO, “The Japan Society of Applied Physics/Organic Molecular Electronics and Bioelectronics”, vol. 11, No. 1 (2000), 86-99).
In view of the foregoing, above-mentioned deterioration can be reduced by improving the current efficiency (luminance generating depending on the electric current), since the necessary amount of electric current to achieve a certain luminance can be reduced. Thus, the current efficiency is an important factor for an organic device in view of the device lifetime, not to mention the power consumption.
However, an organic EL device has a problem with respect to the current efficiency. As mentioned above, the light emission mechanism of the organic EL device is that light is converted by recombination of the injected hole and electron with each other. Therefore, in theory, it is possible to extract at most one photon from the recombination of one hole and one electron, and it is impossible to extract a plurality of photons therefrom. That is, the internal quantum efficiency (the number of emitted photons depending on injected carriers) should be at most 1.
However, in reality, it is difficult even to bring the internal quantum efficiency close to 1. For example, in the case of the organic EL device using the fluorescent material as the luminant, the statistical generation ratio of the singlet excited state (S*) and the triplet excited state (T*) is considered to be S*:T*=1:3. Therefore, the theoretical limit of the internal quantum efficiency is 0.25. (Reference 4: Tetsuo TSUTSUI, “Textbook of the 3rd seminar at Division of Organic Molecular Electronics and Bioelectronics, The Japan Society of Applied Physics” (1993), 31-37). Furthermore, as long as the fluorescent quantum yield from the fluorescent material is not φf, the internal quantum efficiency will be decreased even lower than 0.25.
In recent years, there has been an attempt to bring the theoretical limit of the internal quantum efficiency close to 0.75 to 1 by using phosphorescent materials obtained from the light emission of the triplet excited state. The internal quantum efficiency has been actually achieved exceeding that of the fluorescent material. However, the range of material choice is unavoidably restricted since a phosphorescent material having high phosphorescent quantum efficiency φp should be used. That is caused by that the organic compounds that can release phosphorescent light at room temperature are extremely scarce.
For this reason, as a means for improving the inferiority of the current efficiency of a device, the concept of a charge generation layer was reported in recent years (Reference 5: M. Herrmann, Junji KIDO, “49th Japan Society of Applied Physics and Related Societies” p. 1308, 27p-YL-3 (March 2002)).
The concept of a charge generation layer is described as illustrated in FIGS. 7A-B. FIGS. 7A-B are frame formats of the organic EL device disclosed in Reference 5 that is formed by laminating an anode, an first electroluminescent layer, a charge generation layer, a second electroluminescent layer, and a cathode. Note that the electroluminescent layer (hereinafter, an EL layer) is a layer including an organic compound that can emit light by injecting carriers. In addition, the charge generation layer does not connect to an external circuit and serves as a floating electrode.
In such an organic EL device, when voltage V is applied to the region between the anode and the cathode, electrons are injected to the first EL layer from the charge generation layer and holes are injected to the second EL layer from the charge generation layer, respectively. When seen from the external circuit, holes are moving from the anode to the cathode and electrons are moving from the cathode to the anode (FIG. 6A). However, it can be also seen that both holes and electrons from the charge generation layer are moving in the reverse direction (FIG. 6B), so that carriers are recombined in both of the first EL layer and the second EL layer, and light is generated. In that case, if the current I is flowing, both of the first EL layer and the second EL layer can release photons depending on the amount of current I, respectively. Therefore, such organic EL device have the advantage of releasing two times amount of light by the same amount of current compared to an organic EL device having only one layer. (However, two times or more amount of voltage is needed compared to the organic EL device having only one layer).
In the organic EL device employing such a charge generation layer, the current efficiency can be improved significantly by laminating a number of EL layers. (However, the structure requires several times or more amount of voltage). Thus, in theory, the device lifetime can be expected to be improved along with the improvement of the current efficiency.
However, when the current efficiency is tried to be improved using a charge generation layer, it is required that a number of EL layers should be laminated and the fabricating process become complicated. Accordingly, the partial defect possibility such as a pinhole is increased. Therefore another defects such as the dispersion of each element, the short-circuit of elements, and the like are apt to be caused. That is, the problem may be occurred with the yield of devices though the essential reliability of the device is improved according to improving the current efficiency.